1. Field of the Invention
This invention relates to a multi-functionality television testing signal generator using a digital scheme and, more particularly, to a television signal generator for generating television testing signals for testing transmission paths in a television system and its associated apparatuses.
2. Description of the Related Art
When in transmission paths in a television broadcast system or each constituent circuit in a television set or a video tape recorder are tested to determine whether they are normally operated, a television testing signal having a predetermined waveform is applied to each device to be tested, and it is checked whether a normal test image is displayed on, e.g., a cathode-ray tube in the television.
The waveforms of such television testing signals are defined by various official organizations such as the International Radio Consultative Committee (CCIR) and the Federal Communications Commission (FCC), and are recommended to be used.
As is known, an NTSC television signal constituting one TV picture consisting of 525 scanning lines is roughly divided into a field-blanking period and a picture line period, as shown in FIG. 20. The field-blanking period is constituted by a 3H-equalizing pulse pre-period, a 3H-vertical synchronizing pulse period, a 3H-equalizing pulse post-period, and a black burst period. The picture line period is constituted by video signals 2 divided by horizontal sync signals 1.
The equalizing pulse, the vertical synchronizing pulse, and the black burst (color burst signal) respectively have the waveforms shown in FIGS. 21A, 21B, and 21C. If video signal 2 for a picture line period is a composite signal defined by the CCIR, it is composed of a plurality of elements 3, as shown in FIG. 21D, while the waveforms of the elements 3 are respectively defined.
In a television signal generator for generating such a television testing signal, high signal generation precision is required. The circuit size of a signal generator using analog circuit elements becomes large. In addition, satisfactory stability of output signals cannot be obtained in such a signal generator. For this reason, a television signal generator using a digital technique has been developed.
FIG. 22 is a block diagram showing a schematic arrangement of a conventional television testing signal generator using a digital technique. Referring to FIG. 22, reference numeral 4 denotes a ROM for storing each peak value (data value) converted into a 10-bit digital value. Each peak value is obtained by sampling a plurality of signal waveforms as a construction unit the line period of a television signal on the time base (dividing one line into 3640 equal parts), respectively. One-line data values including black burst data, equalizing pulse data, vertical synchronizing pulse data, testing signal data, etc. are stored in each line of the ROM 4. More specifically, an upper address of an address representing a storage position of each data value represents each line from 1 to 525 lines, whereas a lower address represents a position of each waveform on the time base. Vertical scanning address counter 5 is updated by a clock synchronized with a line frequency (about 15 kHz). An output from vertical scanning address counter 5 designates each output data value of waveform output designating memories 6a, 6b, and 6c of ROM 6. Each of memories 6a to 6c stores upper address information of ROM 4 regarding the output line location of the waveforms shown in FIGS. 21A to 21C. According to this example, three types of output sequences are stored. Every time the output value of vertical scanning address counter 5 is updated, information for designating a line to be output, i.e., the upper address value of a readout data value from ROM 4 is supplied to ROM selector 7. ROM selector 7 supplies only the upper address from one of waveform output designating memories 6a to 6c designated by a waveform selection signal input from operation panel 8. When the line data to be supplied to ROM 4 is designated by the upper address, one-line address counter 9 sequentially designates the lower addresses of the designated line data at a frequency 3640 times the line frequency (about 16 times subcarrier frequency fs), i.e., about 57 MHz. As a result, peak values constituting the waveforms of the designated line data along the time base are sequentially output and supplied to D/A converter 10. Thus, the one-TV-picture (one-frame) television signal designated by operation panel 8 is output from D/A converter 10.
The peak values along the time base obtained by sampling the respective signal waveforms constituting the television testing signal at a predetermined frequency are stored in ROM 4 as the digital values in this manner, thereby greatly improving electrical stability of output television testing signals.
However, in the television signal generator arranged as shown in FIG. 22, the following problems are posed. As described above, since a one-line signal waveform is divided into 3640 equal parts, and each peak value is displayed with ten bits, one-line data is equal to (10 bits.times.3640) words. In addition, since the three types of signals shown in FIGS. 21A, 21B, and 21C are always required, if the number of types of desired video signals 2 is N, ROM 4 requires a memory capacity of EQU 10 bit.times.3.6 K words.times.(3+N).
For example, if number N of types of desired video signals 2 to be output is 100, a total memory capacity of 100.times.3.6 KW.times.10 bits is required provided that the field-blanking period in FIG. 20 is omitted. If 256-K (32.768 KW.times.8-bit) ROMs are used, 14 ROMs are required. Since a large number of memory elements are required in this manner, the manufacturing cost is increased.
In addition, a luminance signal component (Y component), a chrominance signal component (C component), and a sync signal component (SYNC) are included in each video signal 2. These components are separately detected in a television set. Since each peak value (data value) of the waveforms of video signal 2 obtained by mixing these components is converted into 10 bits, and is converted into an analog value by D/A converter 10, sufficient resolution cannot be obtained in D/A converter 10. Therefore, amplitude linearity of a television testing signal D/A-converted with insufficient resolution is degraded. As a result, a large quantization error may be included in the subsequent signal processing operations.
Furthermore, nonlinearity of D/A converter 10 may degrade phase linearity of a television testing signal.
As described above, the conventional digital television testing signal generator has the problem of memory capacity and its related problems mainly because ROMs are used as the waveform memories. With an improvement of a video system to be described later, another problem to be solved has been posed in the conventional generator, i.e., that it is not suitable for a multi-function system.
TV systems have been widely used as high-quality video media. In order to realize a high-quality of video media, IDTV, EDTV, and HDTV are respectively employed as feasible programs. The IDTV has already been put into practice.
According to another trend, the following home video has become commercially available. In this home video, an interface of composite signals is replaced with that of Y/C separation signals so as to realize a wide-band video signal and obtain cross-color and dot interference prevention effects, and the like, thereby providing a clear image.
With these trends towards improvement in image quality, it is a very practical and important theme to obtain a high-quality image by applying various signal processing techniques especially to the existing TV systems.
That is, demands have arisen for realizing a multifunction type TV (video) signal generator capable of flexibly adapting itself to such a theme by utilizing digital techniques. However, a technique using waveform memories has the following limitations.
According to the fundamental principle of a signal generator using the digital techniques, waveform data prestored in a ROM or a RAM is D/A-converted to obtain an output signal. In the generator using the ROM, the output waveform cannot be changed or a user cannot arbitrarily form a waveform. The system using the RAM can perform the above operations. However, if a complex TV waveform is to be formed, problems are posed in terms of an internal realizing means, operability, a backup method for a formed waveform, and the like. According to a method of solving these problems, these operations may be controlled by an external computer, and the function of the system itself is limited to simple memory and output means. In this case, however, the condition for a stand-alone type cannot be satisfied.